Woe betide anyone who calls for a rethink of England's green belt, for they will receive enough opprobrium to gorge a landfill site. Already the epithets are raining down on Helen Phillips for venturing such a suggestion. But when the government's chief adviser on conservation argues that this 70-year old concept needs updating, might she not have a point?

Yesterday's proposals by Ms Phillips and the board of Natural England are based on a pretty simple notion: if the government wants 3m new houses, it needs to plan where they go. Since about two-thirds of that total have yet to be allocated sites, this debate is likely to be more far-reaching than ministers will admit. Because one feature of this debate is politicians' refusal even to enter into a debate over what has become sacrosanct land. The government vows not to alter long-standing "robust rules", while the Conservatives constantly warn of ministers' plans to "let rip with the concrete mixer and... bulldoze the green belt".

Green-belt refuseniks have only two possible responses open to them. One is simply to call off the attempt to provide more homes to those who need them - although that would not stop housebuilders doing what they currently do and nibbling away at the green belt. The other answer is to insist developers utilise brownfield land and build in greater density. This is sensible, and around 60% of the new houses probably will be on brownfield. But that leaves a shortfall in sites - besides which, cities too deserve lungs and not to have their perimeters full of glorified chicken coops.

Politicians are responding to the public's affection for green spaces. Here the debate is prone to misconceptions. Kate Barker's review into land use, commissioned by the Treasury and published last year, featured an opinion poll showing 54% of respondents believed over half of England was developed. The truth is that about 13.5% is - around the same amount that is green belt. And while the name has accrued environmental connotations, the idea behind having a corset of land around big cities was merely to prevent urban sprawl.

The main merit of the green belt today is that it is a crude, but reasonably effective, barrier to a sea of concrete washing over the countryside. We cannot have a frank debate about updating the system without first ensuring adequate safeguards against housebuilders encroaching ever further into green land. The real answer would be a more stringent planning regime. Yet this government has weakened our concrete defence barriers, by giving power to regional development authorities. England's green-belt policy needs to be discussed. But not without stronger planning controls, too.